1. Field of the Invention
The present invention relates to a marine riser system and method of installing the riser and more particularly relates to an integrated, multi-conduit marine riser system to conduct fluids between the marine bottom and the surface.
2. Description of the Prior Art
In producing crude oil and natural gas in offshore areas, floating production systems have been used to transfer large volumes of fluids between subsea installations and the water surface. Floating production systems are becoming more attractive as petroleum production extends to water depths beyond the economic and physical limitation of fixed platforms and to distances beyond the economic limits of pipelines. Most floating production systems have wellheads and special production manifolds on or near the ocean floor and processing equipment and storage facilities on a moored floating vessel. Fluid-handling lines, or conduits, are used to transfer hydrocarbons between the subsea wellheads or manifold systems on the ocean floor and the vessel mounted production equipment.
Many floating production systems use a multi-conduit riser system to transfer fluids from the producing wells to the surface. The multi-conduit riser system should be easy to construct and must safely transport large quantities of petroleum to the surface.
A common multi-conduit production riser design utilizes a number of small diameter satellite conduits, or risers, disposed around a larger diameter central conduit. The satellite conduits are guided through appropriate passages in brackets attached to the central conduit. Movements of the surface vessel caused by wind and sea swell cause bending forces in the central conduit. These bending forces can be reduced by the incorporation of a flexible joint between the lower end of the central conduit and the well. Tensioners located on board the floating vessel can be used to independently tension the central conduit and each of the satellite conduits.
A principal shortcoming of a large conduit surrounded by smaller satellite conduits is that ocean waves and currents may produce vortex shedding forces that subject the riser system to excessive vibration and fatigue stresses. Vortex shedding occurs when water flows at high velocity around a riser. Vortices alternately shed from each side of the riser in a periodic fashion create pulsating forces leading to vibration of the central and satellite conduits. Vortex shedding is a particular problem in a multi-conduit riser system because the small diameter satellite conduits are more susceptible to bending than the larger central conduit.
The vortex shedding forces increase stress within the conduits by inducing vibrations within the conduits. These vibrations produce cyclic bending stresses at the brackets used to fasten the satellite conduits to the central conduit. When the conduits of a multi-conduit riser system vibrate at different frequencies, the system can experience fatigue cracks at the brackets fastening the conduits.
The vortex shedding forces further increase stress within the conduits by increasing the current drag forces bending the conduits. As a current impinges on a conduit, the current drag exerts a bending force against the conduit. This bending force induces stress at the brackets used to fasten the satellite conduits to the central conduit. Drag forces are magnified by vortex-induced vibrations because a vibrating conduit disrupts the current flow more than a stationary conduit does. This disruption increases the frictional forces impeding the current flow. The drag forces are particularly increased by the vibration of conduits in an exposed, multi-conduit riser because of the increased turbulence resulting from fluid flow around a combination of lines. The hydrodynamic forces resulting from this turbulence are extremely difficult if not impossible to accurately predict.
While fairings and other means can be used to suppress vortex-induced vibration of a single riser, there are no adequate means adapted to a multi-conduit riser. A need, therefore, exists for an improved multi-conduit production riser that will withstand the forces imposed by ocean waves and currents, is sufficiently flexible to tolerate vessel surge and sway on the surface of the water, and is easy to install.